1. Field of the Invention
This invention relates to an infrared camera system having an array of sensor element, called Focal Plane Array (FPA), and its design. It also relates to the heat transform in the camera.
The invention also relates to an absorbent/emitting shielding, a shutter without position sensor provided in the system, and data/information regarding different optical elements to be connected to the system. It also relates to the design of the coupling between the FPA and the absorbent shielding.
The camera also comprises a signal processing system connected to the FPA. The processing system is provided with a program to adapt the interpretation of the signals from each detector element to features in the surrounding of the FPA, and means to feed information about amended features in the surrounding to the signal processing system.
2. Description of the Related Art
IR-cameras are usually very expensive devices, since their manufacture is rather complicated and need individual mechanical adjustments. There is a need for inexpensive and exact IR-cameras, particularly of the temperature measuring kind. There is also a need for an IR-camera, which allows the user to exchange parts of it himself and make smaller adjustments.
IR-cameras need to be calibrated. This is particularly true for IR-cameras having an array of sensor elements, such as a focal plane array (FPA), preferably of the un-cooled type, i.e. cameras with a small amount of movable elements. One strives to have as few movable elements as possible and also to have as few elements as possible in the system that are dependent upon the mechanical position particularly of moving parts.
Calibration of an IR camera is commonly done at the manufacturer. Normally, a re-calibration must be done as soon as an exchange of some vital optical elements have been done, or some movable parts have been displaced. Therefore, the camera has up to now been sent to the manufacturer in order to make this re-calibration. This is a drawback for the users of the camera and there is a wish to provide a system, in which the calibration could be made in an easy way, preferably automatically.
Since it is a camera having an IR recording made by a non-moving sensor system the calibration is provided on the signals from the individual sensor elements in the FPA.
To ensure that the image quality and/or temperature measurement accuracy is good enough the infrared camera system has to compensate for several factors associated to the optical concept. For example it can be parameters that compensate for non-uniformity, transmission loss, vignetting, different lens temperatures, spectral characteristics and other features.
Another problem is that IR cameras using any shuttering/flag device to normalize or calibrate the infrared image information from the detector need to know when the shuttering device is in the optical path, protecting the detector from detecting radiation from objects outside the system, to be able to calibrate. The standard solution to this is to have sensors of various kinds detecting the position/location of the shuttering device. Another solution is to wait during a predetermined time after activating the shuttering device. This makes the normalization and calibration time rather long.
The fact that the presented image and temperature values must first be compensated for the different lenses and filters used has several drawbacks. The user of infrared cameras needs to send the system for calibration if he/she gets a new lens or filter.
The need and design of an absorbent shielding in IR-cameras is well known. Un-cooled FPA detector elements have to be kept at a stable temperature, defined within some range, emerging from IR radiation and the internal heat generated by the electronic circuits inside the IR camera. There have always needed to be some heat sink between the FPA and the camera housing in order to get rid of the excessive heat. The heat sink solutions so far are accompanied with mechanical mounting and time consuming alignment difficulties, which earlier had to be made by the manufacturer. The detector and FPA housing are very sensitive for any pressure changes due to heat expansion of the mounting of the heat sink and the camera housing. The changes are detected as flames in the IR image.
The need and design of an absorbent shielding in IR cameras is well known. Unwanted IR radiation outside the focused IR energy from the system optics has to be either redirected away from the detector device or the FPA or to be absorbed by the inner housing by the baffles and absorbent shielding. Various designs exist with either very high demand on the emissivity of the inner surface and/or complicated geometry to eliminate stray light reaching the IR detector or FPA.
One common way to solve the problem with the shielding is to introduce several baffles at calculated intervals between the entrance aperture and the detector entrance window following the shape of the optical bundle. Such baffles are inserted during the manufacturing process and greatly increase the cost of the baffle. The inserted baffles should be infinitely thin since the edges give rise to an additional source of scattering.
There are some known shielding arrangements. U.S. Pat. Nos. 5,227,782 and 5,315,116 describe that any radiation outside the optical bundle could be reflected into an optical cavity and reflected out of the system after several surface reflections predicted by means of ray trace. Approach with advanced geometrical solutions to redirect or retro reflect the stray light back through the optics are described in U.S. Pat. Nos. 5,994,702 and 5,298,752. Approach with advanced absorbing surface cavities and microstructures are described in U.S. Pat. No. 5,196,106.
U.S. Pat. No. 6,144,031 describes a thin-walled shielding in front of an un-cooled infrared sensor, which can be of radiometric type. The shielding is comprised in a vacuum chamber together with the infrared sensor. The optics comprises a relay optic cell, which is fixedly mounted on the camera housing, and an imaging optical assembly, which is detachable, such as an assembly having a short focal length and an assembly having a long focal length can be interchanged. However, in each case the path of marginal rays leading from the relay cell entrance aperture to the FPA is unchanged.
An object according to the invention is to provide high precision IR cameras to a low cost and adapted to be produced in high volumes.
Another object of the invention is to provide IR cameras, which are self calibrated, or easily calibrated by the person handling the camera.
Still another object of the invention is to provide an IR camera, in which its different parts are easily exchangeable.
Another object of the invention is to provide an IR camera, in which it is ensured that the customer does not need to send the equipment away for calibration when he/she has acquired a new lens or filter. This will make it possible for the customer to start using the new lens/filter directly.
Still another object of the invention is to provide an IR camera, which makes the radiometry and the image calibration more accurate.
Another object of the invention is to provide an IR camera, in which the number of components and cost are reduced but which has an improved system performance.
Still another object of the invention is to provide an IR camera having a low cost absorbent shielding suitable for high volume IR cameras without sacrificing any considerable lost of measuring accuracy.
Yet another object of the invention is to provide an IR camera, in which the time consuming mounting and alignment procedure to the camera housing is avoided.
The invention relates to an IR camera comprising:
a. an IR Focal Plane Array comprising a number of detector elements as sensor means;
b. an optical system focusing an object onto said Focal Plane Array;
c. signal processing system connected to said Focal Plane Array;
d. a modular building comprising:
d1. a camera housing provided with said Focal Plane Array and said signal processing system;
d2. an absorbent/emitting shielding device connected to said camera housing; and
d3. an optical focusing system being removably mounted to said shielding device.
The IR-camera further comprise:
program means in said processing system to adapt the signals from said detector elements in said Focal Plane Array to features in surroundings of said Focal Plane Array:
information means to feed information about amended features in said surroundings to said program means in said signal processing system.
The IR-camera could also comprise a Focal Plane Array holding device providing a thermal coupling directly from said Focal Plane Array to the absorbent shielding, and pressing means pressing said holding device against said shielding.
A cavity provides said absorbent shielding. It has a first aperture at one end wall turned to said Focal Plane Array, and a second aperture at another end wall for the beam path from said object to said Focal Plane Array. The cavity has a ratio of depth to width such that all stray light outside the optical path to the Focal Plane Array has to be reflected at least three times inside the cavity before it can go through the first aperture to reach the Focal Plane Array. The cavity is preferably approximately cylindrical, and the dimension of the cavity is at least 1 to 5 and has a radius being at least 3 times the width of any of the apertures. The Focal Plane Array has preferably a small size. A coating with a high absorption coefficient could be provided on an approximately cylindrical inside wall of the cavity. A simple wedge geometry of the inner cylindrical walls could further increase the absorption inside said cavity.
The holding device of the Focal Plane Array is pressed against the shielding, for example with screwed joint in order to provide direct thermal contact between two thermally conducting devices, for example metallic. Thus, according to one aspect of the invention the detector housing is floating in relation to the camera housing, to which it belongs. All the heat exchange is provided through the shielding and the radiation losses through the optics. This is in contrary to earlier solutions, where there has been a heat bridge between the enclosure for the Focal Plane Array and the camera housing in order to remove the excessive heat.
A shutter means may be provided between the optical focusing system and the Focal Plane Array.
By using the IR detector itself to detect when the detector signal has reached a predicted and/or steady state from the time the shutter close signal has been activated, there is no need for position sensors. It makes also a fast calibration since the waiting time from providing the shutter close signal until the steady state of the detector signal is optimal.
This method can be used even with a non-moving part shutter solution.
In one aspect of the invention the lens package or the filter is equipped with a device making it possible for the infrared camera to get the data associated with the lens/filter from the lens/filter itself. The data (information) stored using the device could be, but are not limited to, non-uniformity correction, transmission, vignetting parameters, compensation matrixes, spectral characteristics, lens/filter part number, lens/filter serial number.
When the lens/filter is mounted to the infrared camera for the first time the camera automatically downloads (transfer) or interprets the data from the device integrated in the lens/filter and uses the new data to compensate for the specific characteristics of that lens/filter. The camera can, but does not have to, store the data so that next time the lens/filter is used and identified the data are already stored in the camera and do not need to be downloaded again. However, it is also possible to make the readings of the characteristics of the optics continuously. It is also possible to have the data on a card readable for the camera when inserted in a reading device in the camera.
The method for storing information about the lens/filter can be of any kind for example electronic device such as a memory, optic or magnetic (bar) code, mechanical part that makes it possible for the camera to identify specific information for the lens/filter.
With the data stored in the lens or filter there is no need to send the system for a complementary calibration. Also the customer will be able to acquire new types of lenses and filters that were not designed when he/she acquired the infrared camera. When he/she mounts the lens/filer to the camera for the first time the system will automatically read what type of lens or filter it is and what parameters the image and temperature measurement values should be corrected with.
The lens or filter could also be equipped with a temperature sensor in combination with the device storing the lens/filter data. The temperature sensor will then give the infrared camera information of the lens/filter temperature, which can be used for focus compensation, transmission compensation, which could be different for different temperatures, distance calculations etc.
The modular building of the IR-camera makes is easy both to manufacture and makes it easy to handle by the person handling the IR-camera.
Since all lenses are more or less unique, a solution in which every lens could have its own unique parameters stored with the lens identity so that the camera need not be sent away for calibration is a great benefit for the customer. Customer with more then one camera could save cost when it is possible to have the same lens for several cameras.
A customer who has more then one camera can save cost when different lens/filter can be used on all the cameras, i.e. when the lens/filter is no longer specific for the camera.
In combination with a temperature sensor mounted into the lens/filter the compensation of different parameters can be stored with a reference temperature.
The infrared camera is preferably handheld in order to point to different objects that have thermal anomalies or is to be inspected. The method to use such device to point out faulty parts or areas when performing inspections of electrical installation or buildings is easy to manage.
Both the shielding and shutter arrangements reduce the manufacturing cost for the camera.